I mark with the stamps so I can keep my different alloys separate. I lucked out on a 5 gallon pail of high tin solder that I had to melt down (grrrr on the flux core: nasty stuff) and to keep from making some 75% tin boolits, or heaven forbid give away a bunch of lead to make fishing sinkers out of.
When I have some lead left over in the pot that is 50-50 (half and half pure lead and wheelweights), I don`t have to worry in a few months what the alloy is in that random ingot in the pail.
I have a few different types of lead: pure, high tin content, wheelweight, foundry type, and linotype. The foundry type and linotype are not in ingots, though.
The stamps make it very easy to identify which ingot is which, at a glance, even when the ingots are mixed into a different ammo can. (my preferred storage container: they stack very well in the corner of the shed)
A few minutes extra with the stamps cuts down on confusion in the years to come. Or when buddies want a couple of pounds of lead for their own experiments: By mistake I gave a neighbour pure lead to use in his .308. It wasn`t until after he cast a few hundred boolits and loaded another hundred that we realized pure lead doesn`t like 2000 fps in his gun. Since then I have started marking my lead. (although buddy hasn`t asked for any more lead since...)